(a) Field of the Invention
The present invention relates to a process for preparing a composition of resin having phenolic hydroxyl groups (hereinafter referred to as phenol-base resin) which has excellent thermal properties and does not cause time-dependent deterioration in molding processability and mechanical properties such as impact resistance.
(b) Description of the Prior Art
Phenol-based resin has relatively good properties in curing and processing. Cured products of the resin are excellent in electrical and mechanical properties and are widely used as well-balanced materials for molded materials, laminated products, friction materials such as brake linings, shell molds, cast materials and foamed materials. Thus, phenol-based resin is a valuable material in industry.
Phenol-based resin, however, is substantially brittle. When the resin is used for molded materials or laminated products, cracks are liable to develop in a rapid temperature change such as repeated heating and quenching. In the case of molding metal inserted resin, cracks also develop after molding on the contact surface between the metal and the phenol-based resin, and decrease yield of the product.
When phenol-based resin is used for friction materials such as brake lining and as a binder for shell molding materials, cracks or cutouts are apt to occur due to the hard and brittle property of the phenol-based resin.
In order to improve the above disadvantages of phenol-based resin, processes have been proposed to mix nitrile rubber or acrylate base polymers with the phenol-based resin.
Nitrile rubber or acrylate polymers, however, essentially have lower heat resistance than that of the phenol-based resin. Hence, molded articles or laminated products prepared from the phenol-based resin obtained by the above processes decrease their mechanical strengths in continuous use under severe conditions of temperature, for example, 200.degree. C or above. In the case of using these articles or products for friction materials, the field of use is restricted due to lowering of friction coefficient at high temperatures.
As to processes for overcoming these problems, for example, Japanese Laid-open Patent 251452 (1988) and 230661 (1989) disclose phenol-based resin compositions containing dispersed silicone particles in order to improve resistance to cracking, thermal shock and thermal deterioration.
However, silicone rubber dispersed in the phenol-based resin disclosed above exhibits insufficient effect on improving the above properties because of large particle size and incomplete crosslinking reaction.
In practice, when the crosslinking of silicone rubber is unsatisfactory in the phenol-based resin, uncrosslinked silicone separates on the resin surface, gives adverse effects on the viscosity and flowability of a liquid phenol-based resin composition, and causes problems on a solid phenol-based resin, for example, time dependent decrease in flow property which is a curing characteristic of the resin, mechanical strengths, crack resistance, thermal shock resistance and heat deterioration resistance.